Whereas the abbreviation NZP was originally derived from NaZr.sub.2 (PO.sub.4).sub.3, it has now been formulated as a generic term to refer to not only NaZr.sub.2 (PO.sub.4).sub.3, but also to related isostructural phosphates and silicophosphates having a similar crystal structure. Stated in another way, NZP refers to the particular crystal structure exemplified by NaZr.sub.2 (PO.sub.4).sub.3, not merely to the composition thereof.
NZP materials have been commended for a great variety of applications involving thermal shock resistance and/or low thermal expansion and/or high ionic conductivity and/or the capability of enveloping radiogenic isotopes. Hence, previous studies have indicated that NZP materials are characterized by: (1) a wide range in coefficients of thermal expansion; (2) the presence of superionic conductivity in certain compositions containing alkali metals; and (3) the capability of structurally incorporating particular radionuclides such as .sup.127 Cs and .sup.90 Sr. Those prior studies have been dealt solely with single crystals and polycrystalline ceramics. No definitive study of compositionally similar glasses and, by extension, glass-ceramics has been reported, probably because of the very low solubility of ZrO.sub.2 in phosphate-based glasses. However, because of the recognized unique properties exhibited by NZP materials, the primary objective of the instant invention was to develop glass-ceramic bodies containing NZP-type crystals as the predominant and, preferably, the sole crystal phase.